_field_names field
The _field_names field used to index the names of every field in a document that contains any value other than null. This field was used by the exists query to find documents that either have or don’t have any non-null value for a particular field.
Now the _field_names field only indexes the names of fields that have doc_values and norms disabled. For fields which have either doc_values or norm enabled the exists query will still be available but will not use the _field_names field.
Disabling _field_names
Disabling _field_names is no longer possible. It is now enabled by default because it no longer carries the index overhead it once did.
Support for disabling _field_names has been removed. Using it on new indices will throw an error. Using it in pre-8.0 indices is still allowed but issues a deprecation warning.
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https://www.elastic.co/guide/en/elasticsearch/reference/8.17/mapping-field-names-field.html
